In an acoustic microscope sound is generated in some way and then brought to a focus. The object to be studied is placed at or near the focus and an image of the object can be obtained by moving the object laterally and monitoring the variation of the amplitude and phase of the acoustic waves that are reflected from the object or that are transmitted through the object. Instead of moving the object that is to be imaged, one may hold the object in one place and vary the position of the acoustic focus. The position of the acoustic focus can be changed by moving the position of a lens used to focus the sound.
Conventional acoustic microscopes consist of a transducer to generate sound, and a lens to focus the sound, a coupling medium (usually water) through which the sound propagates to come to a focus, and the sample object. The sound reflected from the object may be collected by the same lens that is used to focus the incident beam or by another lens. To collect sound transmitted through the sample a collection lens is needed on the far side of the sample.
It is usually considered that the first scanning acoustic microscope was built by R. A. Lemons and C. F. Quate, (see “Acoustic Microscope—Scanning Version”, Appl. Phys. Lett. 24, 163 (1974)). This used a continuous acoustic wave of frequency 160 MHz, had a resolution of 10 micron, and worked in the transmission mode.
The resolution of an acoustic microscope is determined by the wavelength of the sound that is used and by the numerical aperture of the lens or lenses that are used. To achieve a high resolution, it is necessary to work at the highest possible frequency with a large numerical aperture. According to Briggs, p. 45, the best resolution obtained with water as the coupling medium is described in the work of B. Hadiomioglu and C. F. Quate, Appl. Phys. Lett. 43, 1006 (1983). They used sound pulses of frequency 4.4 GHz and 3 ns duration, with a lens of numerical aperture 0.73 and a radius of 15 micron. The resolution in the linear mode was 0.37 micron and with non-linearity was 0.24 micron. Non-linearity refers to the effect that if the amplitude of the sound near to the focus is sufficiently large, higher frequency harmonics are generated. The presence of these shorter wavelength components improves the resolution. Note that 3 ns at 4.4 GHz corresponds to only 13 cycles.
When the frequency is increased, the attenuation of the sound in water becomes a severe problem. In water, the attenuation in the GHz frequency range varies as the square of the frequency. At 38° C., human body temperature, the attenuation a per unit distance in water is given by a=0.016 f2 micron−1, where f is the frequency in GHz. In a reflection microscope, it is necessary to have the time duration t of the acoustic pulse less than the round trip time from the lens surface to the sample and back. When sound is generated by an electrically driven piezoelectric transducer, it is very difficult to make t less than a few ns (say 5 ns), and so the acoustic path length has to be at least 8 micron and preferably somewhat longer. A 5 GHz sound wave would be attenuated by 35 dB after traveling 10 micron. Thus, for a given working distance from the lens to the sample object, the attenuation effectively controls the highest frequency that can be used and consequently limits the resolution. Water is the usual choice for a coupling medium because of its low attenuation. There are liquids with lower attenuation (e.g., He, H2, CS2, Hg, Ga), but there are difficulties in working with these materials. For example, He and H2 cannot be used as coupling liquids at room temperature. For biological samples, water is generally the only possible coupling medium.